The present invention relates to a multi-level modulation/demodulation method used in a transmission/reception device used especially in a radio communication system.
FIG. 2 shows a concept of the conventional adaptive modulation method. The adaptive modulation method enables a high throughput by controlling the modulation multi-level number according to the propagation condition such as 64 QAM (Quadrature Amplitude Modulation) (204) when the propagation condition is good, QPSK (Quadrature Phase Shift Keying) (202) when the propagation condition is not good, and otherwise, 16 QAM (203). This method is introduced in an article, Shinya OTSUKI et al “Performance of Modulation Level Controlled Adaptive Modulation Systems” (Electronic Information Communication Society Journal B-II, Vol. J78-B-II No. 6 pp. 435-444., June 1995).
FIG. 3 shows the system configuration of this adaptive modulation method. In a communication device (301) of the transmission side, data to be transmitted is encoded by a coding block. In the radio communication, the convolution coding and the turbo coding are often used as the coding method. However, the coding method is not to be limited to these but other coding methods can also be applied. The coded word which has been coded is input to a QAM (Quadrature Amplitude Modulation) adaptive modulation block (304) and modulated. The baseband signal after the modulation is converted to a radio frequency band by a radio frequency block (305) and transmitted. On the other hand, in a radio communication device (302) of the reception side, the radio signal received via an antenna is converted to a baseband by a radio frequency block (306). The baseband signal is firstly subjected to demodulation processing in a QAM demodulation block (307). The demodulation result (likelihood) subjected to soft decision is input a decoding block (308) and decoded in the decoding block (308).
Next, in this adaptive modulation method, the modulation multi-level number in the communication device of the transmission side should be matched with the demodulation multi-level number in the communication device of the reception side. More specifically, the modulation multi-level number m in the QAM adaptive modulation block (304) of the communication device (301) of the transmission side is decided as follows. Firstly, the propagation condition at the time immediately before the demodulation performed by the reception device (302) (or before the present) is measured by using an S/N measurement block (309). Next, an S/N information creation block (310) creates a signal for transmitting this measurement result from the reception device (302) to the transmission device (301). The information created is modulated in the modulation block (312) and transmitted. The transmission device (301) demodulates this reception signal in the decoding block (313) and acquires the propagation condition such as the S/N information. The propagation condition is judged (316) according to the acquired S/N information and the like. If the propagation condition is good, modulation is performed by multi-level modulation with a large multi-level number, and if the propagation condition is not good, modulation is performed by multi-level modulation with a small multi-level number (304). This series of control is closed as a system, i.e., a feedback system, as shown in (317). It should be noted that in FIG. 3 and after, an example of control of the multi-level number is described in one of the links (for example, downlink) but it can also be applied to the other link (uplink).
As shown in FIG. 3, this adaptive modulation method performs high-speed processing of multi-level number control by the feedback system by following the propagation fluctuation and accordingly, there arises a problem that the feedback information (310) is not coded sufficiently and the characteristic is degraded if this information is erroneously judged due to the noise in the transmission device (301). Moreover, since this adaptive modulation method switches the multi-value numbers by using the feedback system, it is difficult to perform the multi-level number control in a unit shorter than the time required for the feedback. More specifically, as shown in FIG. 4, when the time required for the feedback is (402), if a propagation fluctuation (401) faster than the control cycle (update cycle) occurs, the propagation condition cannot be followed and there is a problem that the characteristic is degraded because transmission is performed with a high multi-level number when the propagation condition is not good and on the contrary, transmission is performed with a low multi-level number when the propagation condition is good. Furthermore, when the adaptive modulation is applied to carriers of the communication method performing simultaneous communication by using a plurality of carriers such as the OFDM (Orthogonal Frequency Division Multiplexing), the feedback information increases in proportion to the number of carriers and there arises a problem that the processing amounts at the transmission device and the reception device become significantly great.